Gas separation process

ABSTRACT

A process for separating a feed gas comprising polar and non-polar gases into a gas mixture enriched in polar gas(es) and a gas mixture depleted in polar gas(es), the process comprising passing the feed gas through a gas separation unit comprising at least two gas-separation modules in order of decreasing selectivity for the polar gas(es), wherein the feed gas entering the gas separation unit comprises 1 to 35 mol % of polar gas(es).

This invention relates to a process for separating a feed gas comprisingnon-polar and polar gases into a gas mixture enriched in polar gases anda gas mixture depleted in polar gases.

It is known to separate gas mixtures into a gas mixture enriched in oneof the components and a gas mixture depleted in that component using agas separation unit comprising gas-separation modules. One such gasseparation unit comprising modules is described in Japanese patentpublication No. 2014-161744 of FUJIFILM Corporation and is illustratedin FIG. 1.

Typically gas-separation units comprise a number of identicalgas-separation modules connected in series and contained in a housing,as illustrated in FIG. 1. Each gas-separation module comprises agas-selective membrane which separates gas into a permeate which passesthrough the membrane and a retentate which does not pass through thatmembrane. The retentate from each module becomes the feed gas for thenext module in the direction of gas flow. The housing typically has afeed inlet, an outlet for the retentate which has passed through all ofthe modules and one or more permeate outlets. The inlets and outlets areusually located on a side wall of the housing near opposite ends of theunit or on an end wall of the unit.

There is a need for an improved process for separating mixturescomprising non-polar and polar gases into a gas mixture enriched inpolar gases and a gas mixture depleted in polar gases. In particular, itis desirable to remove as much polar gas(es) (e.g. CO₂ and/or H₂S) aspossible from gas mixtures comprising non-polar gas(es) (e.g. CH₄) andlarge amounts of polar gases (e.g. CO₂ and/or H₂S) in a quick andselective manner.

According to the present invention there is provided a process forseparating a feed gas comprising polar and non-polar gases into a gasmixture enriched in polar gas(es) and a gas mixture depleted in polargas(es), the process comprising passing the feed gas through a gasseparation unit comprising at least two gas-separation modules in orderof decreasing selectivity for the polar gas(es), wherein the feed gasentering the gas separation unit comprises 1 to 35 mol % of polargas(es).

The term “comprising” is to be interpreted as specifying the presence ofthe stated parts, steps or components, but does not exclude the presenceof one or more additional parts, steps or components.

Reference to an element by the indefinite article “a” or “an” does notexclude the possibility that more than one of the element(s) is present,unless the context clearly requires that there be one and only one ofthe elements. The indefinite article “a” or “an” thus usually means “atleast one”.

As will be understood, “enriched” and “depleted” are relative to thefeed gas which enters the gas separation unit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of the gas-separation unit known fromJapanese patent publication No. 2014-161744.

FIG. 2 is a sectional view of a gas-separation unit used in the processof the present invention.

FIG. 3 is a partial sectional view of a gas-separation module used inthe process of the present invention.

FIG. 4 illustrates part of a process for making gas-separation modulesof the type shown in FIG. 3.

FIG. 1 illustrates the gas separation unit known from Japanese patentpublication No. 2014-161744 and its use. A feed gas (20) comprising CO₂(a polar gas) and CH₄ (a non-polar gas) enters the gas separation unit(1) via baffle (206). The gas separation unit (1) comprises a housing(12) of circular cross section and a series of identical gas-separationmodules (10 a), (10 b), (10 c), (10 d) and (10 e) connected in seriesvia a central pipe (often referred to as a permeate collection tube).Each module comprises end plates (18A) and (18B) and a gas-tight seal(204) which prevents gas from passing to the next module without firstpassing over a spiral-wound gas separation membrane contained within themodule (not shown). Thus the gas-tight seals (204) ensure that theretentate from each module except for the final module is the feed gasfor the next module. The modules each contain a permeate collection tubeinto which gas which has permeated through the membrane may flow. Thenthe permeate gas (22) from the modules (10 a) to (10 e) exits the gasseparation unit (1) through the permeate collection tube and retentategas (24) exits gas separation unit (1) through perforated end plate(208).

FIG. 2 is identical to FIG. 1 except that the gas separation unit (1)comprises a first set of gas-separation modules (10 a), (10 b) and (10c) and a second set of gas-separation modules (11 a) and (11 b), whereineach of the gas-separation separation modules in the second set ofgas-separation modules (11 a) and (11 b) have a lower selectivity forpolar gas(es) than the gas-separation modules (10 a), (10 b) and (10 c)in the first set of gas-separation modules.

FIG. 3 illustrates a gas-separation module (10) of the type typicallyused in the gas-separation unit. A feed gas (20) comprising polargas(es) (e.g. CO₂ and/or H₂S) and non-polar gas (e.g. CH₄) enters thegas-separation module at one end but is unable to enter the permeatecollection tube (14). The feed gas (20) flows through the module whereit contacts a gas separation membrane amongst the various sheet layerspresent in the gas-separation module. Feed gas (20) which permeatesthrough the gas separation membrane enters the permeate collection tube(12) through perforations (12A) and flows through the central permeatecollection tube (12) and exits as permeate gas (22) for collection. Feedgas (20) which is retained by the membrane (i.e. which does not permeatethrough the gas separation membrane) is unable to enter the permeatecollection tube (12) and exits as retentate gas (24) to either becomefeed gas for the next gas-separation module or to exit the gasseparation unit.

For convenience and space efficiency reasons, it is desirable to pack alarge membrane area into a small volume. One technique for packing alarge membrane area into a small volume is to wind a membrane envelopespirally around a perforated permeate collection tube (12) to create awound membrane structure referred to as a ‘module’, as illustrated inFIG. 3.

The modules (10) typically comprise alternate membrane envelopes andpermeation envelopes wound onto the central permeate collection tube(14). The permeate envelopes are rectangular and are closed on threesides and open one side. The open side of the permeation envelopes areglued to the permeate collection tube (14) such that gas which passesthrough the membrane envelope and into the permeate membrane can travelthrough holes (14A) and into the permeate collection tube (14). Incontrast, a closed side of the membrane envelop is glued to the permeatecollection tube and the open sides of membrane envelop is configured toallow feed gas to enter the membrane envelope. The membrane envelopes,permeation envelopes and central permeation tube are thereforeconfigured such that gas entering the module can only gain entry to thepermeate collection tube (14) by passing through the selective membranein the membrane envelope and into the permeation envelope having itsopen side adhered to the gas separation tube and in gas communicationtherewith via the perforations (14A).

The modules (10) may be made by the process illustrated in FIG. 4 wherethe alternate membrane envelopes and permeation envelopes are wound ontothe central permeate collection tube (14) by rotating the tube in thedirection indicated by curved arrow R. The open edge of the permeationmembrane is glued to the gas permeate tube (14) so that gas may passthrough perforations (14A) iand into the tube (14). The envelopes areheld in position and prevented from unwinding by end plates (18A) and(18B).

Modules are placed in a housing which can withstand high pressures togive what is known as a gas-separation unit (1) and illustrated inFIG. 1. The modules are typically cylindrical and comprise two flat,circular parallel end faces (one at each end) and a curved face ofcircular cross-section joining them. Feed spacers are often included inthe membrane envelopes to space the membranes apart and give the feedgas free access to the membrane surface. Permeate spacers are typicallyprovided between the membrane envelopes to guide gas which has permeatedthrough the wound membrane envelope(s) to a permeate collection tube.The envelopes therefore typically comprise outer membrane sheets and aninner feed spacer, e.g. a screen which creates space between themembranes through which feed gas may flow freely.

Large gas treatment plants often use modular banks or skids ofgas-separation units in order to save money on the cost of valves andpiping. High pressure piping and valves required for feedinggas-separation units are relatively more expensive than thelower-pressure conduits employed for the permeate and reject gasstreams, and provision of gas-separation units comprising many modulesreduces the overall number of required pipe connections.

The housing of each gas-separation unit contains the modules (e.g.spiral wound modules) and as gas passes through the length of eachmodule a portion of the gas permeates through the membranes of themodules and is collected in permeate collection tube. The flow volumeand flow velocity of the feed stream progressively fall with downstreamposition, and the concentration of target gas in the remaining feed gasprogressively decreases after passage along each module due to passageof the target gas through the membranes and into the permeate streaminside the permeate collection tube.

The feed gas comprising non-polar gas(es) and polar gas(es) enters atone end of the housing, at a high rate of flow, and travels along thelength of the inside of the housing contacting one side of each module(e.g. either the inside or the outside of the module). The portion ofthe feed gas which does not permeate through the membranes of themodules exits the unit at the opposite end of the housing as a retentatestream which has a lower content of the target gas than the initial feedstream.

The housing is preferably a cylindrical, tube-like structure ofstandardized diameter and may be configured for large modularinstallations comprising many banks or rows of gas-separation units,each unit holding many modules.

Such gas-separation units may comprise plug-type closures or end capsthat may be removed to provide “full-bore” access to the interior of thehousing for installation or replacement of the modules. The modulestypically have a cylindrical design with simple end seals to enable longchains or strings of modules to be connected in series. This facilitatesthe loading and unloading of modules into the unit, and simplifies theconstruction of large capacity gas-separation plants.

The modules optionally comprise couplings, for example, an end flange,typically with O-ring seals, that can be used to join or snap-fitmodules together in series.

An anti-telescope unit typically is placed at each module to support andabsorb axially-directed forces carried by the module shell.

In the gas-separation units of Japanese patent publication No.2014-161744 only one type of module is used having one specificselectivity for a certain gas mixture at same temperature.

Many of the currently available gas-separation units provide goodCO₂/CH₄ selectivity but have poor CO₂ permeance, leading to a slowseparation process. The ‘permeance’ of a membrane is the membrane'spermeability to a particular gas divided by its thickness. Typicallypermeance is expressed in gas permeance units (“GPU”) and can becontrasted with permeability which is a measure of flow rate through amembrane regardless of the membrane's thickness.

When membranes having high polar gas permeance are used ingas-separation units the increased polar gas flux rate through themembrane typically results in a significant reduction in non-polargas/polar gas selectivity. On the other hand, when membranes having highnon-polar gas/polar gas selectivity are used in gas-separation units thepolar gas flux rate and therefore permeance typically reducesignificantly. There is a need for a process which can provide both goodnon-polar gas/polar gas selectivity and good polar gas permeance.Surprisingly the present invention achieves both good non-polargas/polar gas selectivity and good polar gas permeance.

The gas-separation unit comprises at least 2 modules, more preferably atleast 4 modules, especially at least 7 modules. The maximum number ofmodules is not particularly limited, but for practical purposes thenumber of modules in each housing is preferably less than 20, morepreferably less than 15. A typical housing will contain 8 modules.

The gas-separation module(s) having higher selectivity for the polargas(es) preferably have higher selectivity for at least one, morepreferably for at least half, especially for all of the polar gas(es)present in the feed gas.

As examples of non-polar gases there may be mentioned CH₄, ethane,propane and mixtures comprising two or more thereof. The non-polargas(es) preferably is or comprises CH₄.

As examples of polar gases there may be mentioned H₂S and CO₂ andmixtures comprising two or more thereof. The polar gas(es) preferably isor comprises H₂S and/or CO₂, especially CO₂.

Preferably the non-polar gas(es) are or comprise CH₄ and the polargas(es) are or comprise CO₂ and/or H₂S.

The modules may be connected in series, e.g. a single row of modules ormultiple rows of modules in parallel, with each row comprising modulesconnected in series.

Usually two or more of the gas-separation modules may be connected inseries such that permeate gas which passes through each of the modulesother than the final module is fed into the permeate collection tube ofthe next module.

The feed flow rates at the entrance of the housing are preferably in therange of 0.5-1.5 MMSCFD (million standard cubic feet of gas flow perday).

The typical feed of CO₂ molar fractions in the gas mixtures to beseparated are in the range 2%-35% with inlet pressures of 10-150 bar.

Preferred gas-separation modules are spiral-wound gas-separationmodules, e.g. comprising a permeate collection tube and a membraneenvelope wound spirally around the tube to provide a gas-separationmodule comprising two flat end faces.

Typically the modules comprise a curved wall of circular cross-section,in addition to the end faces, which wall meets the two end faces. Forexample the module may have a generally cylindrical shape comprising thetwo (circular) end faces and a wall (e.g. of circular cross-section)joining the two end faces together. The flat end faces may comprise somesurface texture e.g. caused by the edges of the membranes wound spirallyaround the permeate collection tube.

The function of the permeate collection tube (or “tube” for short) is tocollect the permeate gas which has passed through the membranes. Thetube typically comprises perforations which allow permeate gas to flowfrom the exterior of tube to the interior. The perforations are locatedin the tube such that retentate gas cannot enter the tube.

Thus the membrane envelopes are preferably arranged such that thepermeate can flow through perforations and into the tube and theretentate cannot flow through the perforations.

The tube is typically constructed of a rigid material, for example ametal (e.g. stainless steel) or a plastics material. One will usuallyselect a material which is stable to the permeate gas(es).

The tube may have any cross sectional profile, e.g. triangular, square,pentagonal, hexagonal, elliptical or circular, with circular beingpreferred. Tubes having a circular cross-sectional profile are usefulfor providing cylindrical wound membrane structures, e.g. comprising twoparallel end faces and a third face which has a circular cross-sectionand joins with the two end faces.

The membrane envelope typically comprises outer membrane sheets and aninner feed spacer. The membrane sheets are usually rectangular and havetwo long edges and two short edges. Rectangular membrane sheets may befolded in two at the centre, and the feed spacer is located inside thefold, typically against the inside short edge.

The module preferably comprises more than one membrane envelope, e.g. 2to 100, especially 20 to 50 membrane envelopes.

Typically the membrane sheets comprise composite membranes, e.g.comprising a discriminating layer and a porous support. The function ofthe discriminating layer is to preferentially discriminate betweengases, separating a feed gas mixture into a permeate which passesthrough the membrane and a retentate which does not pass through themembrane. The permeate and retentate typically comprise the same gasesas the feed gas mixture, but one is enriched in at least one of thegases present in the feed gas and the other is depleted in that samegas.

The porous support is typically open pored, relative to thediscriminating layer. The porous support may be, for example, amicroporous organic or inorganic membrane, or a woven or non-wovenfabric. The porous support may be constructed from any suitablematerial. Examples of such materials include polysulfones,polyethersulfones, polyimides, polyetherimides, polyamides,polyamideimides, polyacrylonitrile, polycarbonates, polyesters,polyacrylates, cellulose acetate, polyethylene, polypropylene,polyvinylidenefluoride, polytetrafluoroethylene, poly(4-methyl1-pentene) and especially polyacrylonitrile.

The porous support preferably has an average thickness of 20 to 500 μm,preferably 50 to 400 μm, especially 100 to 300 μm.

One may use an ultrafiltration membrane as the porous support, e.g. apolysulfone ultrafiltration membrane, cellulosic ultrafiltrationmembrane, polytetrafluoroethylene ultrafiltration membrane,polyvinylidenefluoride ultrafiltration membrane and especiallypolyacrylonitrile ultrafiltration membrane. Asymmetric ultrafiltrationmembranes may be used, including those comprising a porous polymermembrane (preferably of thickness 10 to 150 μm, more preferably 20 to100 μm) and optionally a woven or non-woven fabric support. The poroussupport is preferably as thin as possible, provided it retains thedesired structural strength.

Typically the discriminating layer is present on one side of the poroussupport or is partially or wholly within the porous support.

Preferred discriminating layers comprise a polyimide, especially apolyimide having —CF₃ groups and optionally carboxylic acid groups.Polyimides comprising —CF₃ groups may be prepared by, for example, thegeneral methods described in U.S. Pat. Reissue No. 30,351 (based on U.S.Pat. No. 3,899,309) U.S. Pat. Nos. 4,717,394 and 5,085,676. Typicallyone or more aromatic dianhydrides, preferably having —CF₃ groups, arecondensed with one or more diamines. The diamine(s) and dianhydride(s)copolymerise to form an AB-type copolymer having alternating groupsderived from the diamine(s) and dianhydride(s) respectively.

Preferably the discriminating layer comprises groups of the Formula (1)wherein Ar is an aromatic group and R is a carboxylic acid group, asulphonic acid group, a hydroxyl group, a thiol group, an epoxy group oran oxetane group:

Preferably the discriminating comprises further a polyimide comprisinggroups as shown in formula (2) wherein n and y are varied between 0 and100%. The higher the y is in percentage versus the n the higher thePI-polymer is in selectivity and the lower in flux (permeance) for a gasmixture comprising non-polar and polar gases.

Optionally there may be a polymeric layer between the porous support andthe discriminating layer, often referred to as a gutter layer. Preferredgutter layers comprise a dialkylsiloxane.

The feed spacer is preferably a screen, e.g. having a large mesh size toallow the feed gas to travel axially along membrane module. In mostinstances, the feed spacer will be utilized, but it is possible toconstruct a module without this component. In general, a feed spacer isformed of any inert material which maintains a space between themembranes.

After the membrane module has been wound, the resultant wound membranestructure may be held in a wound state through the use of restrainingbands, outer wraps, an anti-telescoping device (ATD) secured to thepermeate collection tube or a combination of two or more thereof. Apreferred method of restraining the wound membrane structure (inaddition to using an ATD) is by filament winding, in which a glass fibrefilament dipped in an epoxy resin is wound around the wound membranestructure and cured. The wound membrane structure can then be loadedinto the housing and optionally connected to further modules.

In operation, the feed gas is typically introduced through the gas inletat one end of the gas-separation unit and contacts a perforated baffle(206). The baffle modifies and evens-out the gas flow, which thencontacts the first gas-separation module. The feed gas travels axiallythrough the housing, along the module and into the feed spacer. As thefeed gas encounters the gas-separation module, part of the feed gas (orpermeate) passes through the membrane and into the permeate envelope.After the permeate passes through the membrane, it travels along thepermeate carrier, eventually passing into permeate collection tubethrough openings in the tube. The permeate optionally exits the modulethrough a permeate outlet and the retentate travels axially through thehousing and eventually exist the housing through a retentate outlet.

The target gas may be non-polar gas(es) (e.g. CH₄) or polar gas(es)(e.g. CO₂ and/or H₂S). Recently the separation and capture of polargases, especially CO₂, has attracted attention in relation toenvironmental issues (global warming). Typically the feed gas is passedthrough a first set of gas-separation modules and then through a secondset of gas-separation modules, wherein each of the gas-separationmodules in the second set of gas-separation modules has a lower polargas selectivity than the gas-separation modules in the first set ofgas-separation modules, especially lower CO₂ and/or H₂S selectivity. Thefirst and the second set of gas-separation modules preferably eachcomprise at least two or three gas-separation modules.

Preferably all of the gas-separation modules in the second set ofgas-separation modules comprise gas separation membranes having anapolar gas/non-polar gas (e.g. αCO₂/CH₄) of 5 to 25 and all of thegas-separation modules in the first set of gas-separation modulescomprise gas separation membranes having an apolar gas/non-polar gas(e.g. αCO₂/CH₄) of 20 to 50. The polar gas/non-polar gas selectivity ismeasured at the conditions of each experiment which will be discussed inthe following paragraphs.

In a preferred embodiment:

-   (i) the gas-separation modules of the first set of gas-separation    modules have identical polar gas/non-polar gas selectivity to each    other;-   (ii) the gas-separation modules of the second set of gas-separation    modules have identical polar gas/non-polar gas selectivity to each    other; and-   (iii) the gas-separation modules of the second set of gas-separation    modules have lower polar gas/non-polar gas selectivity than the    gas-separation modules of the first set of gas-separation modules.

Preferably the feed gas enters the first gas-separation module at atemperature below 60° C.

Typically the gas-separation modules form part of a gas separation unit,the gas separation unit further comprising a housing which contains thegas separation modules, the gas separation modules are connected inseries and the housing further comprises a gas inlet and at least twogas outlets.

In a preferred embodiment the gas separation unit is free fromtemperature controlling devices and gas pressure controlling devices. Inthis way the cost of the gas separation is reduced and so are themaintenance requirements and potential for mechanical failure.

Preferably the feed gas is fed into the first of the gas-separationmodules at a pressure of 5 to 80 bar, more preferably 10 to 70 bar andespecially 20 to 60 bar.

Preferably the feed gas is fed into the first of the gas-separationmodules at a flow rate of 0.1 to 2 million standard cubic feet per day(“MMSCFD”), more preferably 0.5 to 2 MMSCFD and especially 0.5 to 1.5MMSCFD. Preferably the feed gas is fed into the first of thegas-separation modules at a temperature of 20 to 60° C., more preferably30 to 55° C. and especially 30 to 50° C.

In a preferred embodiment the gas separation unit comprises a series ofmodules not all of which have the same polar gas/non-polar gasselectivity, each module comprising a gas-selective membrane whichseparates gas into a permeate which passes through the membrane and aretentate which does not pass through that membrane, wherein gas flowsthrough the unit such that retentate from each module except for thefinal module becomes the feed gas for the next module and wherein thegas-separation modules are arranged such that the polar gas selectivityof the first module through which the gas flows is lower than the polargas selectivity of the final module through which the gas flows. Theseries of modules preferably comprises a first set of gas-separationmodules and a second set of gas-separation modules, wherein each of thegas-separation modules in the second set of gas-separation modules has alower polar gas selectivity than all of the gas-separation modules inthe first set of gas-separation modules and wherein the gas separationunit and the modules are arranged such that the retentate from eachmodule except for the final module is the feed gas for the next module.The invention is further illustrated by the following Examples.

EXAMPLES AND COMPARATIVE EXAMPLES

The following materials were used to prepare the membranes sheets:

-   PAN is a porous support polyacrylonitrile L10 ultrafiltration    membrane from GMT Membrantechnik GmbH, Germany.-   PET is HW 2503 polyester and epoxy resin 75:25% from Hornwood.-   UV9300 is SilForce™ UV9300 from Momentive Performance Materials    Holdings. This is thermally curable copolymer comprising at least 3    epoxy groups and linear polydimethyl siloxane chains. Furthermore,    this copolymer cures rapidly when irradiated with UV light in the    presence of a photo-initiator.-   I0591 is 4-isopropyl-4′-methyldiphenyliodonium    tetrakis(pentafluorophenyl) borate (C₄₀H₁₈BF₂₀1) from TCI (a    photo-initiator which is free from mono-epoxy compounds).-   Ti(OiPr)₄ is titanium (IV) isopropoxide from Dorf Ketal Chemicals.-   n-Heptane is n-heptane from Brenntag Nederland BV.-   MEK is 2-butanone from Brenntag Nederland BV.-   CH is cyclohexanone from Brenntag Nederland BV.-   PI1 is (6FDA-TeMPD)_(n)-(6FDA-DAB)_(y) of the following structure,    wherein n is 20% and y is 80% (% being % mol/mol) obtained from    FUJIFILM Corporation:

-   PI2 is as defined above for PI1 except that n is 80% and y is 20% (%    being % mol/mol), obtained from FUJIFILM Corporation.-   PE Interfoil is Mylar™ A50, obtained from Dupont (a gas-impermeable    polyester sheet of 50 μm thickness.-   PP is Naltex 1717_90 Polypropylene.    Preparation of Membranes    Stage a) Preparation of a Partially Cured Polymer 1 (“PCP Polymer    1”)

A solution of a PCP Polymer 1 was prepared by heating the componentsdescribed in Table 1 together for 105 hours at 95° C. The resultantsolution of PCP Polymer 1 had a viscosity of about 64,300 mPa·s whenmeasured at 25° C.

TABLE 1 Ingredients used to prepare PCP Polymer 1 Ingredient Amount (w/w%) UV9300 75 Ti(OiPr)₄ 1.5 n-Heptane 23.5Stage b) Preparation of Radiation Curable Composition 1 (“RCC1”)

Portions of the solution of PCP Polymer 1 obtained in stage a) abovewere cooled to 20° C., diluted with n-heptane and then filtered througha filter paper having an average pore size of 2.7 μm. The remainingingredients indicated in Table 2 below were added to make RCC1 havingthe formulation described in Table 2 below.

TABLE 2 Ingredient Type Ingredient RCC1 Inert solvent n-Heptane (w/w %)84.9 MEK (w/w %) 1.6 PCP Polymer PCP Polymer 1 (w/w %) 13.3Photo-initiator I0591 (w/w %) 0.2Stage c) Preparation of Compositions Used to Form a Discriminating Layer

Compositions DSL1 up to DSL2 were prepared by mixing the componentsshown in Table 3 and filtering the mixtures through a filter paperhaving an average pore size of 2.7 μm.

TABLE 3 Ingredients used to prepare Discriminating layers DSL1 and DSL2DSL1 DSL2 PI1 (w/w %) 2.0 — PI2 (w/w %) — 2.0 CH (w/w %) 6.0 6.0 MEK(w/w %) 92.0 92.0Stacie d) Preparation of Membrane 1 or Membrane 2

Membrane 1 and 2 were prepared having the layers described in Table 4below.

The radiation-curable composition RCC1 was applied to a PAN substrate(step a)) at a speed of 10 m/min by a meniscus dip coating. Thecomposition was then cured by irradiating (step b)) using a Light HammerLH10 from Fusion UV Systems fitted with a D-bulb at an intensity of 16.8kW/m (70%) to give a substrate carrying a gutter layer of 300 nm drythickness. A discriminating layer was formed on the gutter layer usingthe compositions DSL1 for Membrane 1 up or DSL2 for Membrane 2 asindicated in Table 4, using a meniscus type coating T 10 m/min coatingspeed. The discriminating layer of Membrane 1 comprised PI11 andMembrane 2 comprised PI2.

TABLE 4 Membranes 1 and 2 Example Membrane 1 Membrane 2Radiation-curable Composition RCC1 RCC1 Coating speed (m/min) 10 10Coating amount (ml/m²) 3 3 Dry layer thickness of gutter layer (nm) 300300 Discriminating layer composition DSL2 DSL1 Coating amount (ml/m²)8.4 8.4 Dry layer thickness of discriminating 120 120 layer (nm)αCO₂/CH₄ 15 30Stacie e) Preparation of Gas Separation Module 1 (“M1”)Permeate Carrier Tube

A tube of internal diameter 47 mm and external diameter 50 mm, made fromstainless steel Grade 316, was cut to a length of 1 m. Holes of diameter4 mm were drilled through the tube wall to give an aperture ratio of 15%(i.e. the holes occupied 15% of the surface area of the permeate carriertube.

(e1) Permeate Envelopes

A rectangular, gas-impermeable sheet made of PE interfoil (600 mm×600mm) was sandwiched between two rectangular sheets of permeate carriermade of PET (900 mm×900 mm. The gas impermeable sheet was positioned atthe centre of the short edge of the permeate carrier sheets and fixedthere using an adhesive to give a permeate envelope. This was repeated afurther 20 times to give 21 permeate envelopes, each comprising permeatecarrier—gas-impermeable sheet—permeate carrier.

(e2) Membrane Envelopes

A rectangular sheet (900 mm×1,800 mm) of Membrane 1 was folded around afeed spacer sheet made of PP (900 mm×900 mm). The feed spacer sheet waspositioned at the centre of the short edge, inside the fold of themembrane sheet and fixed there using an adhesive to give MembraneEnvelope 1. This was repeated a further 21 times to give 22 MembraneEnvelopes, each comprising Membrane 1—feed spacer—Membrane 1.

(e3) Preparation of Gas-separation Module (M1)

The membrane fold of a first membrane envelope (as described in (e2)above) was glued onto a permeate carrier tube. The long sides of apermeate envelope were then glued to the permeate carrier tube to form agas-tight seal, with the permeate envelope (prepared as described in(e1) above). This process was repeated until all to 22 membraneenvelopes and 21 permeate envelopes were adhered to the permeate carriertube in an alternate manner. The envelopes were then wound spirally ontothe permeate carrier tube in the manner illustrated in FIG. 4 and theenvelopes were held in position and prevented from unwinding by endplates (18A) and (18B). This provided a cylindrical, gas-separationmodule comprising alternate membrane envelopes and permeate envelopeshaving two parallel, essentially circular end faces and an overallcircular cross-sectional profile. Plastic bands were applied to theresultant module to prevent the wound envelopes from unwinding to givegas separation Module M1.

Stage f) Preparation of Gas Separation Module 2 (“M2”)

Gas separation module 2 was prepared in exactly the same way as gasseparation module 1 except that in place of Membrane 1 there was usedMembrane 2.

Stage g) Preparation of Gas Separation Units

Gas separation units comprising gas-separation modules M1 and/or M2 inthe order indicated in Table 5 were prepared by loading the modules inthe desired order into a housing as illustrated in FIG. 1. Each housingcomprised a feed gas inlet and gas outlets for permeate gas andretentate gas, located on the side wall of the housing as shown in FIG.1.

The feed flows indicated in Table 5 were as measured one day after useof the relevant gas separation unit had begun. In all experiments thetemperature of the feed gas was 40° C. and the housing was not subjectedto any extrinsic pressure or temperature control.

Gas Permeance/Selectivity of the Membranes

A) Gas Permeance

The permeance of non-polar gas and polar gas through the compositemembranes M1 and M2 were measured at 40° C. and gas feed pressure of6000 kPa using a gas permeation cell with a measurement diameter of 3.0cm and a feed gas composition of 13 v/v % polar gas (CO₂) and 87 v/v %non-polar gas (CH₄).

The flux of each gas was calculated based on the following equation:Q _(i)=(θ_(perm) ·X _(Perm,i))/(A·(P _(Feed) ·X _(Feed,I) P _(Perm) ·X_(Perm,i)))wherein:

Q_(i)=Flux of each gas (m³(STP)/m²·kPa·s)

θ_(Perm)=Permeate flow (m³(STP)/s)

X_(perm,I)=Volume fraction of each gas in the permeate

A=Membrane area (m²)

P_(Feed)=Feed gas pressure (kPa)

X_(Feed,i)=Volume fraction of each gas in the feed

P_(Perm)=Permeate gas pressure (kPa)

STP is standard temperature and pressure, which is defined here as 25.0°C. and 1 atmosphere (101.325 kPa).

(B) Selectivity of the Membranes

The selectivity (apolar gas/non-polar gas) for the composite membraneswas calculated from Q_(CO2) and Q_(CH4) calculated above, based onfollowing equation:αpolar gas/non-polar gas=Q _(CO2) /Q _(CH4)(C) Selectivity and permeance of the pas separation unit

The polar gas permeance (GPU) and apolar gas/non-polar gas (selectivity)of the whole gas separation module configurations described in Table 5were back-calculated from test data according to the formulationdescribed in “Calculation Methods for Multicomponent Gas-separation byPermeation” (Y. Shindo et al., Separation Science and Technology, Vol.20, Iss. 5-6, 1985). The membrane surface area of each module which usedto calculate the permeance was 29 m². The feed pressure was varied andthe mol % of CO₂ in the feed gas mixture was varied too as described inTable 5, where the inlet CO₂ partial pressure is also shown.

The mole % of polar gas and no-polar gas in the feed and permeate weremeasured by gas chromatography.

Selectivity (αpolar gas/non-polar gas) was calculated from Q_(CO2) andQ_(CH4) as described in (B) above.

TABLE 5 Configuration of the Modules (gas flowing Gas Feed polar Totalleft to right and M1 feed gas Feed polar gas Polar gas αpolar gas/non-No. of having lower polar gas pressure [mol CO₂/mol partial pressureFeed flow Permeance polar gas Example modules selectivity than M2) [bar]mixture] [bar] [MMSCFD] [GPU*] (selectivity) CEx1 6 M1/M1/M1/M1/M1/M1 600.05 3 0.5 51 10 CEx2 6 M2/M2/M2/M2/M2/M2 60 0.05 3 0.5 33 25 Ex1 6M2/M2/M1/M1/M1/M1 60 0.05 3 0.5 46 15 CEx3 6 M1/M1/M1/M1/M2/M2 60 0.05 30.5 44 14 CEx4 8 M1/M1/M1/M1/M1/M1/ 60 0.05 3 1.0 60 13 M1/M1 CEx5 8M2/M2/M2/M2/M2/M2/ 60 0.05 3 1.0 36 27 M2/M2 Ex2 8 M2/M2/M1/M1/M1/M1/ 600.05 3 1.0 55 17 M1/M1 CEx6 8 M1/M1/M1/M1/M1/M1/ 60 0.05 3 1.0 53 15M2/M2 CEx7 6 M1/M1/M1/M1/M1/M1 60 0.15 9 1.0 65 15 CEx8 6M2/M2/M2/M2/M2/M2 60 0.15 9 1.0 33 28 Ex3 6 M2/M2/M1/M1/M1/M1 60 0.15 91.0 52 23 CEx9 6 M1/M1/M1/M1/M2/M2 60 0.15 9 1.0 45 23 *GPU = 7.5 * 10⁻⁹Nm³/m² * kPa * s

The invention claimed is:
 1. A process for separating a feed gascomprising CO₂ and non-polar gases into a gas mixture enriched in CO₂and a gas mixture depleted in CO₂, the process comprising passing thefeed gas through a gas separation unit comprising at least twogas-separation modules in order of decreasing selectivity for the CO₂,wherein (i) the feed gas entering the gas separation unit comprises 1 to35 mol % of CO₂; (ii) each gas-separation module comprises a gasseparation membrane; (iii) the feed gas is passed through a first set ofgas-separation modules and then through a second set of gas-separationmodules, wherein each of the gas-separation modules in the second set ofgas-separation modules has a lower CO₂ selectivity than all of thegas-separation modules in the first set of gas-separation modules; and(iv) all of the gas-separation modules in the second set ofgas-separation modules comprise gas separation membranes having anαCO₂/CH₄ selectivity of 5 to 25 and all of the gas-separation modules inthe first set of gas-separation modules comprise gas separationmembranes having an αCO₂/CH₄ selectivity of 20 to
 50. 2. The processaccording to claim 1 wherein the second set of gas-separation modulescomprises at least three gas-separation modules.
 3. The processaccording to claim 1 wherein the gas-separation modules of the first setof gas-separation modules have identical CO₂ selectivity to each other.4. The process according to claim 1 wherein the gas-separation modulesof the second set of gas-separation modules have identical CO₂selectivity to each other.
 5. The process according to claim 1 whereinthe feed gas enters a first gas-separation module of the at least twogas-separation modules at a temperature below 60° C.
 6. The processaccording to claim 1 wherein the gas-separation modules are spiral woundgas-separation modules.
 7. The process according to claim 1 wherein thegas separation unit further comprises a housing which contains the gasseparation modules, the gas separation modules are connected in seriesand wherein the housing further comprises a gas inlet and at least twogas outlets.
 8. The process according to claim 7 wherein the gasseparation unit is free from temperature controlling devices and gaspressure controlling devices.
 9. The process according to claim 1wherein the gas-separation modules comprise gas separation membraneshaving polyimide discriminating layers comprising —CF₃ groups andoptionally carboxylic acid groups.
 10. The process according to claim 1wherein the feed gas is fed into a first of the at least twogas-separation modules at a pressure of 20 to 60 bar.
 11. The processaccording to claim 1 wherein the feed gas is fed into a first of the atleast two gas-separation modules at a flow rate of 0.5 to 1.5 MMSCFD.12. The process according to claim 1 wherein the feed gas is fed into afirst of the at least two gas-separation modules at a temperature of 30to 50° C.
 13. The process according to claim 1 wherein the gasseparation unit comprises a series of modules not all of which have thesame CO₂ selectivity, each module comprising the gas separation membranewhich separates gas into a permeate which passes through the gasseparation membrane and a retentate which does not pass through thatmembrane, wherein gas flows through the gas separation unit such thatretentate from each module except for a final module becomes the feedgas for a next module and wherein the gas-separation modules arearranged such that the CO₂ selectivity of a first module through whichthe gas flows is higher than the CO₂ selectivity of the final modulethrough which the gas flows.
 14. The process according to claim 13wherein the series of modules comprises a first set of gas-separationmodules and a second set of gas-separation modules, wherein each of thegas-separation modules in the second set of gas-separation modules has alower CO₂ selectivity than all of the gas-separation modules in thefirst set of gas-separation modules and wherein the gas separation unitand the gas-separation modules are arranged such that the retentate fromeach module except for the final module is the feed gas for the nextmodule.
 15. The process according to claim 1 wherein the non-polargas(es) are or comprise CH₄.
 16. The process according to claim 13wherein the non-polar gas(es) are or comprise CH₄.